Field of the Invention and Related Art Statement
The present invention relates to a method of manufacturing products of ceramics, and particularly relates to a method of manufacturing products having a cylindrical outer surface such as a translucent alumina tube for use in a high-voltage electric discharge lamp and a valve for use in automobile parts.
In a manufacturing process of the products of ceramics having a cylindrical shape or a columnar shape, granulated powder is molded by isostatic pressing such as hydrostatic pressing to obtain a green body. An outer surface of the green body is ground before sintering in order to make the surface smooth. In a grinding process of the green body, the outer surface thereof is generally ground by using a bite or a wheel around which a abrasive is provided. By grinding the outer surface of the green body with the bite or the wheel, a final product having uniformity in thickness and a smoothness on the outer surface thereof can be obtained. Particularly, in a manufacturing process of a translucent alumina tube for use in a high-voltage electric discharge lamp, it is necessary to make the outer surface of the green body smooth in order to obtain an alumina tube as the final product having a predetermined translucency. For this purpose, grinding with the bite o the wheel was generally conducted to finish the green body of the alumina tube.
FIGS. 1A and 1B are cross- and longitudinal-sectional views showing a green body 1 of the translucent alumina tube which has a cylindrical shape. According to the known grinding method for grinding the outer surface of the green body 1, in order to obtain a high roundness of the cylindrical body and a uniformity of thickness, the outer surface la of the green body 1 is ground by using the bite or the wheel to a degree that an unevenness formed on the outer surface is completely removed. That is to say, the outer surface of the green body 1 should be ground not only up to the minimum outer diameter of the cylindrical body but also up to the line shown by the reference D2 in FIGS. 1A and 1B, taking a margin for grinding into consideration.
However, such high roundness and uniformity of the thickness are not required to manufacture the product such as the translucent alumina tube for the high-voltage electric discharge lamp. In such tubes, if the sufficient translucency is obtained, the roundness of the body and the uniformity of the thickness do not effect to the efficiency of the tube. Therefore, only a smoothness of the outer surface 1a of the green body 1 is required to finish the green body. If the outer surface of the product such as the translucent alumina tube for use in the high-voltage electric discharge lamp is ground by the bite or the wheel, the utilization coefficient of the material of the product would be decreased. Furthermore, when the product is made by a brittle material such as ceramics, the grinding with the bite or the wheel would cause a crack on the outer surface 1a of the green body 1 and then the molded body 1 would be broken before or after sintering. The starting material of the translucent alumina tube is so expensive that the problem of the cracking was very serious in manufacturing the translucent alumina tube.